Energy recovery from shock absorbers through a novel compact electro-hydraulic system architecture

Riding a road vehicle involves only 10-16% of the fuel energy to overcome the resistance from friction and air drag, while the rest contributes to a massive ecological and economic damage. From an energy balance point of view, a great amount of energy is dissipated as vibrations at the shock absorbers level. In this paper, a hydraulic regenerative shock absorber, able to recover and convert the vibration energy caused by road profiles is designed and manufactured by exploiting off-the-shelf components to reduce R&D costs, and its overall maximum efficiency is measured. The prototype is composed of a twin tube electronically controlled shock absorber and of an electro-hydraulic unit (also called regenerative valve) rigidly connected to it. The main relations ruling the dynamics of a regenerative damper are discussed and the main features of the regenerative valve are designed to comply with size, performance, and safety requirements. Parametric relations are illustrated and properly managed to justify the choice of a dedicated electric motor. Performances are assessed through simulations and experimental tests, while reliability is confirmed through a thermal analysis that reveals no thermal issues. The prototype reaches a maximum overall harvesting efficiency of 11.7% for an intermediate value of the external electric load at a 500mm/s rod speed and a maximum electric power in output of 75W for a high external electric load at 1000mm/s rod speed. Efficiency is strongly hindered at low values of rod speed because of predominant friction effects. It is also demonstrated the possibility to modify the damping curve by varying the electrical characteristics of the circuit connected to the electric machine. The compact design of the RSA, with a mass of only 5.8kg, allows its installation into the suspension of a B-class SUV with a minimum impact on the current damper structure.